1. Field of the Invention
This invention relates to light measuring devices, and particularly to light integrating circuits for converting quantities of light into electrical signals. The invention is especially directed to light measuring devices for use in cameras.
2. Description of Related Art
The dynamic range of a conventional light integrating circuit, that is a light measuring device using an integrating circuit, is too small to respond efficiently to changes in the amount of light impinging thereon. For example, if the voltage of such a device varies between 18 mV and 2V, the ratio of the highest voltage to the lowest voltage is 2.sup.7. This is the equivalent of only 7 steps in a camera aperture, or more generally, only 7 steps in the APEX value system.
When a camera uses through the lens (TTL) light metering to adjust a flash unit, the light integrating circuit of the measuring system requires a dynamic range of 10 APEX steps just to accommodate film sensitivities from ISO 6 to ISO 6400. Ten more APEX steps are necessary for changing the value of a photographic lens from F 1.0 to F 32. Thus, changing the film sensitivity and aperture value independently requires a dynamic range of 20 APEX steps.
Consequently, conventional light integrating circuits used with flash devices to control the amount of light shed on the object, have a dynamic range which is too narrow for this purpose.
The dynamic range of a light integrating circuit may be widened by logarithmically compressing the photo current produced in the light measuring photo sensor, such as a photodiode. In this manner, each doubling of the amount of light striking the photodiode results in a linear increase in integrated voltage. Thus, a change in output voltage from 18 mV to 1.8V, presents 100 doublings, or a dynamic range of 100 APEX steps.
However, it has been discovered that light integraters using logarithmic compression are accurate only for large light quantities and becomes less accurate at smaller light quantities.